The present invention relates generally to particle time-of-flight measurement systems and more particularly to particle detector devices used in conjunction with such systems capable of detecting particles at low energy and with high timing resolutions.
It will be appreciated by those skilled in the art that particle time-of-flight measurement techniques are useful in both ion back scattering systems, as shown, for example, in Applicant's U.S. Pat. No. 5,026,988 issued Jun. 25, 1991, and entitled "Method and Apparatus for Time-of-Flight Medium Energy Particle Scattering", and in mass spectrometry such as that described in U.S. Pat. No. 4,490,610 issued Dec. 25, 1984, to the United States of America. In many such systems, microchannel plate detectors are used in order to generate start and stop pulse signals for determining the time-of-flight of particles which enter the detector. In prior art microchannel plate detectors, the rear plate surface or anode of the microchannel plate is operated at or near DC ground potential with the front plate surface or cathode typically biased at -2,000 volts DC. This creates a voltage differential across the microchannel plate which has an accelerating effect on the particles which approach the plate. Such a biasing scheme also allows the microchannel plate detector to be coupled to the signal processor of a time-of-flight measurement system using conventional BNC connector having a grounded shield side.
Unfortunately, the biasing schemes of prior art microchannel plate detectors do not allow them to be used effectively in detection of low energy electrons and negative ions, because the negative potential at the cathode repels these particles. Consequently, some in the prior art have attempted to modify the DC biasing scheme on microchannel plate detectors such that the cathode of the microchannel plate is operated at or near ground, with a substantially more positive DC potential applied to the anode. However, when one uses such a "grounded cathode" biasing scheme, the high DC voltages must be isolated from the BNC connectors in the system because they are not rated to withstand a 2,000 volt potential across the signal and shield sides. A conventional prior art response to this problem has been to use simple capacitive decoupling to separate the pulse signals from the detector from the underlying DC bias voltage. This, however, creates an additional problem when the microchannel plate detector is to be operated with high timing resolutions, typically better than 500 picoseconds. Using conventional capacitive decoupling at the interface between the microchannel plate detector and the measurement system signal processor produces distortion and reflection of the pulse signal due to an impedance mismatch at the interface. This distortion and reflection of the signal pulse will produce inaccuracies in the measurement of the particle time-of-flight.
What is needed, then, is a microchannel plate detector device which can accurately detect low energy electrons and negative ions with high timing resolution, and which can easily be coupled to conventional time-of-flight measurement systems using standard connectors. Such a device is presently lacking in the prior art.